1 / 49

Today

Today. Move on to …. Hmm… Lets look at this again. Not drawn to scale . L1/L2 cache: 64 B blocks. ~4 MB. ~4 GB. ~500 GB. Disk. L1 I-cache. L2 unified cache. Main Memory. 32 KB. CPU. Reg. L1 D-cache. Throughput:. 16 B/cycle. 8 B/cycle. 2 B/cycle. 1 B/30 cycles. Latency:.

raja
Télécharger la présentation

Today

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Today • Move on to …

  2. Hmm… Lets look at this again Not drawn to scale L1/L2 cache: 64 B blocks ~4 MB ~4 GB ~500 GB Disk L1 I-cache L2 unifiedcache Main Memory 32 KB CPU Reg • L1 • D-cache Throughput: 16 B/cycle 8 B/cycle 2 B/cycle 1 B/30 cycles Latency: 3 cycles 14 cycles 100 cycles millions

  3. Roadmap • Memory & Data • Integers/FP • Machine code • Assembly programming (x86) • Procedures/Stacks • Caches • Processes (today!) • Virtual Memory • Memory Allocation • Java vs C • (maybe) a little more on processor HW

  4. Processes and control flow • Is branches/calls the only way we can get the processor to “go somewhere” in a program? • What is a process?

  5. Control Flow • Processors do only one thing: • From startup to shutdown, a CPU simply reads and executes (interprets) a sequence of instructions, one at a time • This sequence is the CPU’s control flow (or flow of control) Physical control flow <startup> inst1 inst2 inst3 … instn <shutdown> time

  6. Altering the Control Flow • Up to now: two mechanisms for changing control flow: • Jumps and branches • Call and return Both react to changes in program state • Insufficient for a useful system: difficult to react to changes in system state • user hits “Ctrl-C” at the keyboard • user clicks on a different application’s window on the screen • data arrives from a disk or a network adapter • instruction divides by zero • system timer expires • How do we deal with the above? Are branches/calls sufficient?

  7. Altering the Control Flow • Up to now: two mechanisms for changing control flow: • Jumps and branches • Call and return Both react to changes in program state • Insufficient for a useful system: difficult to react to changes in system state • user hits “Ctrl-C” at the keyboard • user clicks on a different application’s window on the screen • data arrives from a disk or a network adapter • instruction divides by zero • system timer expires • System needs mechanisms for “exceptional control flow”!

  8. Exceptional Control Flow • Exists at all levels of a computer system • Low level mechanisms • Exceptions • change in control flow in response to a system event (i.e., change in system state, user-generated interrupt) • Combination of hardware and OS software • Higher level mechanisms • Process context switch • Signals – you’ll hear about these in CSE451 and CSE466 • Implemented by either: • OS software (context switch and signals) • C language runtime library (nonlocal jumps)

  9. Exceptions • An exception is transfer of control to the operating system (OS) in response to some event (i.e., change in processor state) • Examples: div by 0, arithmetic overflow, page fault, I/O request completes, Ctrl-C • How does the system know where to jump to? User Process OS exception event I_current exception processing by exception handler I_next • return to I_current • return to I_next • abort

  10. Interrupt Vectors • Each type of event has a unique exception number k • k = index into exception table (a.k.a. interrupt vector) • Handler k is called each time exception k occurs Exception numbers code for exception handler 0 Exception Table code for exception handler 1 0 1 code for exception handler 2 2 ... ... n-1 code for exception handler n-1

  11. Asynchronous Exceptions (Interrupts) • Caused by events external to the processor • Indicated by setting the processor’s interrupt pin(s) • Handler returns to “next” instruction • Examples: • I/O interrupts • hitting Ctrl-C at the keyboard • clicking a mouse button or tapping a touch screen • arrival of a packet from a network • arrival of data from a disk • Hard reset interrupt • hitting the reset button • Soft reset interrupt • hitting Ctrl-Alt-Delete on a PC

  12. Synchronous Exceptions • Caused by events that occur as a result of executing an instruction: • Traps • Intentional • Examples: system calls, breakpoint traps, special instructions • Returns control to “next” instruction • Faults • Unintentional but possibly recoverable • Examples: page faults (recoverable), protection faults (unrecoverable), floating point exceptions • Either re-executes faulting (“current”) instruction or aborts • Aborts • Unintentional and unrecoverable • Examples: parity error, machine check • Aborts current program

  13. Trap Example: Opening File • User calls: open(filename, options) • Function open executes system call instruction int • OS must find or create file, get it ready for reading or writing • Returns integer file descriptor 0804d070 <__libc_open>: . . . 804d082: cd 80 int $0x80 804d084: 5b pop %ebx . . . User Process OS exception int pop open file returns

  14. Fault Example: Page Fault int a[1000]; main () { a[500] = 13; } • User writes to memory location • That portion (page) of user’s memory is currently on disk • Page handler must load page into physical memory • Returns to faulting instruction • Successful on second try 80483b7: c7 05 10 9d 04 08 0d movl $0xd,0x8049d10 User Process OS exception: page fault movl Create page and load into memory returns

  15. Fault Example: Invalid Memory Reference int a[1000]; main () { a[5000] = 13; } • Page handler detects invalid address • Sends SIGSEGV signal to user process • User process exits with “segmentation fault” 80483b7: c7 05 60 e3 04 08 0d movl $0xd,0x804e360 User Process OS exception: page fault movl detect invalid address signal process

  16. Exception Table IA32 (Excerpt) http://download.intel.com/design/processor/manuals/253665.pdf

  17. Processes • Definition: A process is an instance of a running program • One of the most important ideas in computer science • Not the same as “program” or “processor” • Process provides each program with two key abstractions: • Logical control flow • Each program seems to have exclusive use of the CPU • Private virtual address space • Each program seems to have exclusive use of main memory • Why are these illusions important? • How are these illusions maintained?

  18. Processes • Definition: A process is an instance of a running program • One of the most important ideas in computer science • Not the same as “program” or “processor” • Process provides each program with two key abstractions: • Logical control flow • Each program seems to have exclusive use of the CPU • Private virtual address space • Each program seems to have exclusive use of main memory • How are these Illusions maintained? • Process executions interleaved (multi-tasking) • Address spaces managed by virtual memory system – next course topic

  19. Concurrent Processes • Two processes run concurrently (are concurrent) if their instruction executions (flows) overlap in time • Otherwise, they are sequential • Examples: • Concurrent: A & B, A & C • Sequential: B & C Process A Process B Process C time

  20. User View of Concurrent Processes • Control flows for concurrent processes are physically disjoint in time • However, we can think of concurrent processes as executing in parallel (only an illusion?) Process A Process B Process C time

  21. Context Switching • Processes are managed by a shared chunk of OS code called the kernel • Important: the kernel is not a separate process, but rather runs as part of a user process • Control flow passes from one process to another via a context switch… (how?) Process A Process B user code context switch kernel code time user code context switch kernel code user code

  22. fork: Creating New Processes • int fork(void) • creates a new process (child process) that is identicalto the calling process (parent process) • returns 0 to the child process • returns child’s process ID (pid) to the parent process • Fork is interesting (and often confusing) because it is called oncebut returns twice pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); }

  23. Understanding fork Process n pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); }

  24. Understanding fork Process n Child Process m pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid = m

  25. Understanding fork Process n Child Process m pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid = m

  26. Understanding fork Process n Child Process m pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid = m pid = 0

  27. Understanding fork Process n Child Process m pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid = m pid = 0 pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } pid_tpid = fork(); if (pid== 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); } Which one is first? hello from parent hello from child

  28. Fork Example #1 • Parent and child both run same code • Distinguish parent from child by return value from fork • Start with same state, but each has private copy • Including shared output file descriptor • Relative ordering of their print statements undefined void fork1() { int x = 1; pid_tpid = fork(); if (pid == 0) { printf("Child has x = %d\n", ++x); } else { printf("Parent has x = %d\n", --x); } printf("Bye from process %d with x = %d\n", getpid(), x); }

  29. Fork Example #2 • Both parent and child can continue forking void fork2() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("Bye\n"); }

  30. Bye L1 Bye Bye L0 L1 Bye Fork Example #2 • Both parent and child can continue forking void fork2() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("Bye\n"); }

  31. Fork Example #3 • Both parent and child can continue forking void fork3() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("L2\n"); fork(); printf("Bye\n"); }

  32. Bye L2 Bye Bye L1 L2 Bye Bye L2 Bye Bye L0 L1 L2 Bye Fork Example #3 • Both parent and child can continue forking void fork3() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("L2\n"); fork(); printf("Bye\n"); }

  33. Fork Example #4 • Both parent and child can continue forking void fork4() { printf("L0\n"); if (fork() != 0) { printf("L1\n"); if (fork() != 0) { printf("L2\n"); fork(); } } printf("Bye\n"); }

  34. Bye Bye Bye L0 L1 L2 Bye Fork Example #4 • Both parent and child can continue forking void fork4() { printf("L0\n"); if (fork() != 0) { printf("L1\n"); if (fork() != 0) { printf("L2\n"); fork(); } } printf("Bye\n"); }

  35. Fork Example #4 • Both parent and child can continue forking void fork5() { printf("L0\n"); if (fork() == 0) { printf("L1\n"); if (fork() == 0) { printf("L2\n"); fork(); } } printf("Bye\n"); }

  36. Bye L2 Bye L1 Bye L0 Bye Fork Example #4 • Both parent and child can continue forking void fork5() { printf("L0\n"); if (fork() == 0) { printf("L1\n"); if (fork() == 0) { printf("L2\n"); fork(); } } printf("Bye\n"); }

  37. exit: Ending a process • void exit(int status) • exits a process • Normally return with status 0 • atexit()registers functions to be executed upon exit void cleanup(void) { printf("cleaning up\n"); } void fork6() { atexit(cleanup); fork(); exit(0); }

  38. Zombies • Idea • When process terminates, still consumes system resources • Various tables maintained by OS • Called a “zombie” • That is, a living corpse, half alive and half dead • Reaping • Performed by parent on terminated child (horror movie!) • Parent is given exit status information • Kernel discards process • What if parent doesn’t reap? • If any parent terminates without reaping a child, then child will be reaped by init process • So, only need explicit reaping in long-running processes • e.g., shells and servers

  39. ZombieExample void fork7() { if (fork() == 0) { /* Child */ printf("Terminating Child, PID = %d\n", getpid()); exit(0); } else { printf("Running Parent, PID = %d\n", getpid()); while (1) ; /* Infinite loop */ } } • ps shows child process as “defunct” • Killing parent allows child to be reaped by init linux> ./forks 7 & [1] 6639 Running Parent, PID = 6639 Terminating Child, PID = 6640 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6639 ttyp9 00:00:03 forks 6640 ttyp9 00:00:00 forks <defunct> 6641 ttyp9 00:00:00 ps linux> kill 6639 [1] Terminated linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6642 ttyp9 00:00:00 ps

  40. void fork8() { if (fork() == 0) { /* Child */ printf("Running Child, PID = %d\n", getpid()); while (1) ; /* Infinite loop */ } else { printf("Terminating Parent, PID = %d\n", getpid()); exit(0); } } Non-terminatingChild Example • Child process still active even though parent has terminated • Must kill explicitly, or else will keep running indefinitely linux> ./forks 8 Terminating Parent, PID = 6675 Running Child, PID = 6676 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6676 ttyp9 00:00:06 forks 6677 ttyp9 00:00:00 ps linux> kill 6676 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6678 ttyp9 00:00:00 ps

  41. Synchronization!

  42. wait: Synchronizing with Children • int wait(int *child_status) • suspends current process until one of its children terminates • return value is the pid of the child process that terminated • if child_status!= NULL, then the object it points to will be set to a status indicating why the child process terminated

  43. HC Bye HP CT Bye wait: Synchronizing with Children void fork9() { int child_status; if (fork() == 0) { printf("HC: hello from child\n"); } else { printf("HP: hello from parent\n"); wait(&child_status); printf("CT: child has terminated\n"); } printf("Bye\n"); exit(); }

  44. wait() Example • If multiple children completed, will take in arbitrary order • Can use macros WIFEXITED and WEXITSTATUS to get information about exit status void fork10() { pid_t pid[N]; int i; int child_status; for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) exit(100+i); /* Child */ for (i = 0; i < N; i++) { pid_t wpid = wait(&child_status); if (WIFEXITED(child_status)) printf("Child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("Child %dterminated abnormally\n", wpid); } }

  45. waitpid(): Waiting for a Specific Process • waitpid(pid, &status, options) • suspends current process until specific process terminates • various options (that we won’t talk about) void fork11() { pid_t pid[N]; int i; int child_status; for (i = 0; i < N; i++) if ((pid[i] = fork()) == 0) exit(100+i); /* Child */ for (i = 0; i < N; i++) { pid_t wpid = waitpid(pid[i], &child_status, 0); if (WIFEXITED(child_status)) printf("Child %d terminated with exit status %d\n", wpid, WEXITSTATUS(child_status)); else printf("Child %d terminated abnormally\n", wpid); }

  46. execve:Loading and Running Programs Stack 0xbfffffff Null-terminated environment variable strings • intexecve( char *filename, char *argv[], char *envp) • Loads and runs • Executable filename • With argument list argv • And environment variable list envp • Does not return (unless error) • Overwrites process, keeps pid • Environment variables: • “name=value” strings Null-terminated commandline arg strings unused envp[n] = NULL envp[n-1] … envp[0] argv[argc] = NULL argv[argc-1] … argv[0] Linker vars envp argv argc

  47. execve: Example envp[n] = NULL “PWD=/homes/iws/luisceze” envp[n-1] “PRINTER=ps581” … “USER=luisceze” envp[0] argv[argc] = NULL “/usr/include” argv[argc-1] “-l” … “ls” argv[0]

  48. Summary • Exceptions • Events that require non-standard control flow • Generated externally (interrupts) or internally (traps and faults) • Processes • At any given time, system has multiple active processes • Only one can execute at a time, however, • Each process appears to have total control of the processor + has a private memory space

  49. Summary (cont’d) • Spawning processes • Call to fork • One call, two returns • Process completion • Call exit • One call, no return • Reaping and waiting for Processes • Call wait or waitpid • Loading and running Programs • Call execl(or variant) • One call, (normally) no return

More Related